• Question: How do airplanes stay in the air?

    Asked by itch to Allan, Diva, Nathan on 5 Jul 2012.
    • Photo: Allan Pang

      Allan Pang answered on 5 Jul 2012:

      There are four forces involved in flying a plane.

      (1) Lift force – This force pushes the plane upward. The wings of the plane is important in this aspect. The shape allows to create the lift force as air moves across them.

      (2) Weight force – Countering the lift force is the weight of the plane, which got to do with the gravitational pull of Earth. It is crucial to have a plane balanced on its front and back.

      (3) Thrust and motion force – To maintain the plane on air, it needs to have thrust force, provided by propellers.

      (4) Drag force – a countering force of the thrust, which slows down the plane.

    • Photo: Diva Amon

      Diva Amon answered on 5 Jul 2012:

      Hey itch,

      My office at the Natural History Museum in London overlooks the area where planes begin their approach to land in Heathrow airport. That means that a plane flies by in the distance almost every minute and every time one catches my eye, I just find myself thinking that it really is the most fascinating thing – the fact that this huge lump of metal can fly so gracefully through the air!

      Allan is exactly right – 4 forces keep planes up in the air – lift, weight, thrust and drag.

      Lift pushes the plane up. This is helped by the shape of the wings.
      Weight pulls the plane toward Earth but planes are built so that their weight is spread from front to back keeping the plane balanced.
      Thrust moves the airplane forward. Engines give thrust to planes as it keeps air going over the wings.
      Drag slows the plane. Airplanes are designed to let air pass around them with less drag.

      All four of these forces have to act together to keep the plane in the air!

    • Photo: Nathan Langford

      Nathan Langford answered on 5 Jul 2012:

      Hi itch,

      This is an interesting question, but with a surprising answer. We are often taught that the shape of the wings is the critical factor in making them fly. The idea is that by having a curved top side and a flat bottom side, the air has to travel at different speeds across the two surfaces, moving faster around the curved side, because it has farther to go, and this creates a lower pressure above the wing and a corresponding lift force.

      But it turns out that the main source of lift for a plane is much simpler. While the shape of the wings is very important for the aerodynamics and stability of the plane, the main lift is simply provided by the fact that the wings are tilted. It operates exactly like a normal house fan. As the plan moves forward through the air, the air hits the tilted wings and is deflected downwards. But because momentum has to be conserved, if the air is deflected downwards, then the plane is deflected upwards. In effect, the plane bounces up off the air.

      In order to take off the ground, planes therefore need to reach the right speed, but you’ll also see them playing a couple of other tricks to maximise this lift effect. For example, on commercial planes, if you’re sitting near the wing, you’ll see them extend some flaps on the back side of the wing further down to increase the amount of deflection of the air and therefore increase the lift. You’ll probably notice that, as it speeds up, the plane will tilt back on its back wheels before lifting off. This happens when there is enough lift to rotate the plane (imagine riding a bicycle while holding an umbrella), but not yet enough to lift it off the ground. But this has an extra advantage, because the extra tilt further increases the lift.

      You might be able to see a problem here, however. As the plane flies higher and higher, the air gets thinner, and if there’s less air, then there’s also less lift. But since most aircraft fly at up to around 10km altitude, and this is much less than the radius of the Earth which is around 6400km, the weight of the aircraft won’t have reduced very much. That means that to travel at higher altitudes, the planes also need to be travelling faster. Fortunately, the drag would also be less at these altitudes, so this is easier to achieve.

      So, in summary, I guess I’d describe things slightly differently from Allan and Diva. I would say the following. When it’s flying, the plane’s thrust accelerates the plane to the speed where its thrust exactly balances the drag (because the drag increases with speed), so now the plane is flying at a constant speed. It is then the planes movement which combines with the wing tilt to provide enough lift to overcome the weight force of the place. Again, the plane needs to fly at an altitude and speed so that the the lift force exactly balances the plane’s weight.

      Here’s an interesting random little factoid about planes – did you know that one of the US supersonic fighter jets is leaks fuel when it’s on the ground, but not when it is in the air? It is in fact designed to do this, because it is deliberately built to be loose fitting. That’s because when the aircraft is flying, it heats up so much due to friction with the air that the metal pieces of the hull expand and seal up the gaps to be tight fitting… and if it were tight fitting to start with, this would damage the plane!

      Cheers,
      Nathan.

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